How long can a flex duct run be?

Most codes limit flex duct runs to 14 feet, though shorter is better for airflow.

Should I use insulated or non-insulated?

Use insulated flex duct in unconditioned spaces like attics and crawlspaces.

Flex DuctGenerating image…

HVAC & Ventilation

Ductwork

Budget: $0.50-$1.00/ft (non-insulated) | Mid-range: $1.00-$2.00/ft (R-6 insulated) | Premium: $2.00-$3.50/ft (R-8 insulated)

Flex Duct: Flexible HVAC Ductwork for Residential Systems (2026)

Prepared by Lumberama's Building Materials Team

|Last updated May 25, 2026|Sources & methodology

Flex duct is a flexible, lightweight ductwork option used to connect HVAC supply outlets to the main trunk line. It consists of a wire helix covered in plastic and wrapped with insulation and a vapor barrier. Flex duct is popular for residential retrofits and tight spaces where rigid duct is impractical.

Key Features

Lightweight and easy to install; Available in insulated and non-insulated versions; Comes in 4" to 16" diameters; R-4.2 to R-8 insulation values; Flexible routing around obstacles

Specifications

Diameters: 4"

10"

12"

14"

16" | Length: 25 ft standard | Insulation: R-4.2

R-6

R-8 | Max temp: 250°F | Material: Polyester with wire helix

Applications

Connecting HVAC supply registers to trunk lines; Residential heating and cooling distribution; Attic and crawlspace installations; Retrofit HVAC projects; Branch runs from main ductwork

Installation Overview

Cut to length, attach to collar with duct clamp and mastic sealant. Support every 4-5 feet with straps. Avoid kinks and excessive bends to maintain airflow. Keep runs as short and straight as possible.

Pros & Cons

Pros

✓ Easy to install in tight spaces; Lightweight and flexible; Lower cost than rigid duct; Reduces noise transmission

Cons

✗ Higher air resistance than rigid duct; Can sag if not properly supported; Prone to kinking which restricts airflow; Shorter lifespan than metal duct

Top Brands

Master Flow

Dundas Jafine

Speedi-Products

Code Requirements

Must comply with IRC M1601 for duct installation. Maximum length varies by local code. Must be supported per manufacturer specs. UL 181 listed required.

Buying Tips

Always buy insulated flex duct for unconditioned spaces; Size your duct diameter to match the register and HVAC design; Look for UL 181 listed products for code compliance

Frequently Asked Questions

Related Guides

Home Improvement

Ductwork Guide: Types, Cost & Installation

Everything you need to know about HVAC ductwork for your home.

Read article